1. Field of the Invention
This invention relates to a device for detecting the position of a movable member on the basis of a variation in electrostatic capacity resulting from the relative movement of electrodes.
2. Related Background Art
A lens barrel mounted on a camera or the like is provided with a movable barrel holding a lens unit therein and moved in the direction of the optical axis relative to a fixed barrel for magnification change, focusing, etc., and recently the control of a lens system using the position data of said movable barrel (i.e., the position data of the lens unit) has been done. Therefore, various systems for exactly knowing the position of this movable barrel have been proposed, and among them, as an apparatus which is compact and which enables position detection by non-contact between the fixed barrel and the movable barrel, there is known one as shown, for example, in Japanese Laid-Open Patent Application No. 62-262009 wherein paying attention to the fact that electrodes are provided on the fixed barrel and the movable barrel, respectively, in opposed relationship with each other and in non-contact with each other to form a variable capacitor and the electrostatic capacity of this capacitor is varied by the fixed barrel and the movable barrel being moved relative to each other, the position of the movable barrel relative to the fixed barrel, i.e., the position of the lens, is detected from the electrostatic capacity on each occasion.
In the apparatus of the above-described construction, however, the position of the movable barrel (lens) relative to the fixed barrel has been known only from the information of the relative position, and a complicated circuit construction for knowing the absolute position of the movable barrel has been required. Further, of the pair of electrodes forming the variable capacitor, for example, the electrode on the fixed barrel side need be as long as the total amount of movement, and this has hampered the compactness of the device.
Also, in a position detecting device, particularly a lens barrel, when a fixed barrel (a lens) is moved in a fixed barrel in the direction of the optical axis, the imaging magnification of the lens is generally varied by the movement of the lens and sensitivity varies depending on the respective positions of the lens and thus, there are required different position detection accuracies depending on the position of the movable barrel, but the above-described prior-art device is of a construction in which the relation between the position of the movable barrel relative to the fixed barrel and the electrostatic capacity of the variable capacitor is always linear as shown in FIG. 10 of the accompanying drawings. This has led to the problem that there cannot be obtained an accurate position detection accuracy conforming to the position of the movable barrel.